采用分子动力学方法研究了具有不同孪晶界密度的< 111>晶向孪晶Ag纳米线在拉伸载荷下的形变行为,讨论了孪晶界对纳米线力学强度的影响,并分别阐明具有不同孪晶界密度Ag纳米线的塑性形变机理.结果表明,与单晶Ag纳米线的强度进行对比可知,基于不同的形变模式,孪晶界的引入对纳米线可以起到弱化作用或者强化作用.以孪晶片层厚度的倒数(1/TBS)作为临界值,当1/TBS小于0.2 nm-1时,孪晶界作为位错源,表现为对纳米线的弱化作用;当1/TBS大于0.2 nm-1时,孪晶界阻碍位错运动表现为强化作用.强化作用机理分为两种:当1/TBS介于0.2到0.5 nm-1时,形变机理以孪晶界和位错相互作用为主,断裂开口均在纳米线内部产生,随着位错增殖形成孔洞,进而向四周蔓延;当1/TBS大于0.5 nm-1时,孪晶界发生迁移以容纳位错活动,位错不断增殖穿过孪晶界形成剪切带,进而导致纳米线的颈缩.由孪晶界密度不同引起的强化作用和弱化作用均随温度升高而减弱.
This study investigated the deformation behavior of < 111> twin Ag nanowires with differing parallel twin boundary (TB) densities under tensile loading via molecular dynamics (MD) simulations. The effect of TB density on the ultimate stress of nanowires is discussed, and the plastic deformation mechanisms of nanowires are illustrated. The results show that, in contrast to a single crystalline nanowire with the same size, the introduction of the TB can strengthen or soften nanowires through individual deformation modes, which indicates that there exists a critical twin boundary space (TBS) (where the value of the critical 1/TBS is 0.2 nm-1). Below 0.2 nm-1, softening occurs, whereby TBs become the source of dislocations. Above 0.2 nm-1, TBs impede dislocation movement, which results in a strengthening effect. The strengthening mechanisms are divided into two types. When 1/TBS ranges from 0.2 to 0.5 nm-1, the TB-dislocation interaction is the controlling factor. Fracture opening appears within the nanowires, and voids form, with dislocation multiplication, and then spread to the surrounding regions. When 1/TBS is greater than 0.5 nm-1, TBs migrate to accommodate dislocation activity. Dislocations increase and transfer across the TBs. Shear banding is activated during the process, which contributes to the necking of nanowires. The strengthening and weakening effects caused by differences in TB density decrease with increasing temperature